On the role of advection for the net exchange of CO
2at a subalpine grassland in complex terrain
M. Galvagno, G. Wohlfahrt, E. Cremonese, U. Morra di Cella
Environmental Protection Agency of Aosta Valley - ARPA VdA Institute of Ecology - University of Innsbruck
EGU General Assembly, 10 April 2013
• At mostFLUXNETsites, the net ecosystem CO2exchange is evaluated by means of eddy covariance (EC) using a set of instruments on a single tower.
• Assumptions: flat, horizontally homogeneous terrain and stationary conditions.
• Nighttime problem: low turbulent mixing, stable stratification, advective fluxes.
→Underestimation of nighttime net CO2exchange
Mass Balance Terms NEE=Fc+Fs+Fha+Fva
• Eddy covariance measurements are challenging in non-ideal terrains, wheremountain ecosystemsare often naturally situated.
• No information exist on the role of advection at sites withshort canopies
Study site
• Torgnon: Northwestern Italian Alps (Aosta Valley)
• Subalpine grassland (2160 m asl)
• EC measurements since 2008
• Maximum canopy height is 20 cm.
• Not located on a steep slope
• heterogeneous microtopography
Study site
NE winds during nighttime SSW winds during daytime
DEM 20 cm resolution Isolines 2 m
∆ elevation ∼ 30 m
Methods
Four control volumeswhere sequentially investigated during the growing season 2012:
1. Eddy covariance:
- 3D sonic anemometer (CSAT3) - Open-path IRGA (LI7500) placed at1.65 m agl - additional 2D anemometer placed at0.90 m agl
- 10 Hz data processed according to commonly accepted procedures
- Planar-fit method (Wilczak et al, 2001)
2. Profiling system and Chambers:
- Two vertical CO2profiles - Three measurement levels each (0.30, 0.80, 1.65 m agl)
- 30 meters uphill and downhill the EC tower along the different transects - Three ecosystem respiration automated chambers (LI8100, LICOR)
Experimental scheme
Data processing: mass balance terms
Eddy flux
Fc=ρa0 w0c0
Storage term
Fs=R1.65 0
∂c
∂tdz
Horizontal advection Fha=R1.65
0 u(z)∂c(z)∂x dz Vertical advection
Fva=wh1.65(ch1.65− [c])
Results: Eddy fluxes
Results: mass balance terms - mean diurnal variation
Results: mass balance terms - nighttime values
How to correct the nighttime problem?
It was not possible to identify an objective friction velocity threshold
Alternative solutions
1 Using data when the maximum of Fc+Fsis observed in theeveningto develop relationships between NEE and independent variables, such as soil temperature (Van Gorsel et al 2007)
2 To add a constant advective offset to Fc+Fs
3 It appears that there are places, even in this kind of complex topography, where Fc+Fscaptures most ofthe actual nighttime respiration
Conclusions
The main findings of this study are:
1 Fc+Fsconsiderably underestimates nighttime ecosystem respiration as measured by the automated ecosystem chambers.
2 Advection measurements indicate that horizontal, and to a lesser degree, vertical advection are important terms of the full mass balance during nighttime at the grassland site. During daytime advection appears to play a negligible role.
3 The NEE calculated by taking into account advection generally closely resembles nighttime ecosystem respiration as measured with chambers.
4 For two of the control volumes the order of magnitude of NEE computed with the mass balance approach (Fc+Fs+Fha+Fva) was not compatible with biotic fluxes measured by respiration chambers (as previously described for other sites e.g.
Aubinet et al 2010).
5 Ongoing work: testing the best correction approach
6 future 3D Experiments are planned to improve advection measurements.
THANKS FOR YOUR ATTENTION
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